Field of the Invention
The present invention generally relates to a system and method for thermal management of electronic modules and chip scale packages. More particularly the invention relates to providing heat sink mounting methods that decouple the circuit board from a thermomechanically driven deformation, induced by thermal or power cycles, while maintaining required liner shock robustness.
Description of the Related Art
An electronic product such as a computer system is built by integrating a diverse set of components, each made from different set of materials. FIG. 1 shows an assembled computer with a microprocessor unit 1 which is attached to an organic substrate 2 which in turn is attached to a printed circuit board (PCB) 3. Lead free solder interconnects 9 are used to electrically link the above components.
High performance computers are usually built using a ceramic based substrate with multiple processors and memory modules, situated adjacent to the processors. Due to high heat dissipated in the microprocessor unit 1, thermal performance becomes critical to achieving a reliable computer system. To provide efficient heat removal various types of heat sinks are used in practice. Most cost effective heat sinks are air cooled, but for high heat dissipation, liquid or vapor cooled heat sinks are utilized.
A heat sink (HS) 4 typically removes the heat generated from a processor die (e.g., microprocessor unit 1) through a thin thermal interface material (TIM) 8. A preload is imparted on the TIM 8, for example with a preload mechanism 19, in order to keep the thermal conduction thickness to the minimum, typically at about 25 μm. FIG. 1, for example, shows an air cooled system. The laminate polymer material (e.g., printed circuit board 3) and metallic heat sinks 4 are known to have different coefficient of thermal expansion.
Due to power or thermal cycling, discussed subsequently, a computer system is subjected to differential expansion and contraction between individual components. The heat sink 4 and laminate primarily produce a thermomechanically induced strain in critical components. Of particular interest is the strain induced at the edge of the TIM 8 and within solder interconnects 9 that can lead to tearing or fatigue failure respectively, thus negatively impacting the reliability of a computer system.
FIG. 1 corresponds to a microprocessor unit 1 that is made of a silicon die with a substantial foot print (25×25 mm). In this case the heat sink 4 also has to have a substantial foot print in the X-Y plane. Because of the wider foot print and relatively large mass, the heat sink 4, in this case, is mounted rigidly on four solid posts 5, one on each corner. It is noted that in order to illustrate additional elements only one of the four solid posts 5 is illustrated in FIG. 1, but the solid posts (mounting posts) are in fact located in each of the four corners (e.g., as illustrated in FIG. 5 which will be discussed subsequently).
In another application, several chip scale packages (CSPs) 201, providing auxiliary functions such as regulating voltage, may be arranged in-line as shown in FIG. 2. In this case, the thermal management is achieved by a single heat sink 204 covering multiple devices. In the in-line layout, the TIM 208 can extend over the CSP 201. Since the foot print of a CSP 201 in this case is relatively small (6×4 mm), the heat sink 204 tends to be narrow and long, primarily extending along one axis. Typically the heat sink 204 in the in-line mounting is held by two riveted joints 205 placed at the ends. Observe that a solid mounting post or a riveted joint 205 does not allow substantial relative motion between the heat sink 201 and the PCB 203. This condition will be referred to as “no-slip” joint or boundary condition.
Based on the two real life electronic packages that have been described, a heat sink mounting can be either in-plane (X-Y) (e.g., see FIGS. 1 and 5) or in-line (along X or Y) (e.g., see FIG. 2).
Computer systems and data centers are turned on and off regularly which gives rise to thermal cycles. Also, while under power-on-state, an intermittent work load could drive a computer in to and out of active mode from idle mode, generating a highly transient temperature condition. In both cases, differential thermal expansion of components is driven by the thermal and power cycles, causing cyclic strain within critical components. In order to guarantee reliability of parts that are used to build a computer system, rigorous accelerated tests to accentuate field conditions are applied at the development stage by power and thermal cycling the system, as shown in FIG. 3. The ramp rate and dwell times within, for example a 30 minute cycle, are important parameters in determining the reliability of a package.
Once the thermal goals are met by suitable choice of a heat sink, a computer system must be designed to withstand linear shipping shock. Hence, the challenge is to support a heat sink, typically 2 to 3 kg in mass for a high end module, arranged on top of a microprocessor unit (also referred to as a die) without substantially straining the TIM.
FIG. 4 shows a schematic side view of a heat sink 404 supporting a microprocessor 401 and a memory module 412. Any relative motion between the heat sink 404 and the die (e.g., microprocessor 401 or memory module 412) could strain the TIM material 408, and excess strain could lead to tearing of the TIM 408. In addition to linear shock, the products can also be subjected to inadvertent rotational shock. Hence, the heat sink mounting must provide robustness against shock induced damage of TIM 408 or any other vulnerable component.
FIG. 4 also shows a shock pulse in X-direction applied to the PCB 403. To minimize shear strain in TIM 408 the heat sink 404 must be rigidly mounted using high stiffness posts 405 at four corners of the HS 404 so that relative motion between PCB 403 and HS 404 is minimized. FIG. 4 also includes a ceramic substrate 402 and a stiffener plate 414.
FIG. 5 shows a plan view and the location of the four mounting posts 405 that support HS 404. A solid post tends to have a large stiffness against bending, and it certainly prevents large relative motion during a shock which is a positive attribute. However, during power on-off cycle or during thermal cycling, the heat sink 404 made of either aluminum or copper expands at a different rate than the PCB 403. It is not only due to difference in coefficient of thermal expansion (CTE) between HS 404 and PCB 403, but also due to difference in thermal conductivity and heat capacity that determine the time constant of transient heat spreading. During the power ramp up the HS 404 pushes the four mounting posts 405 along the expansion vector (usually along the diagonal) and forces the printed circuit board assembly to bend through the lever arm provided by the mounting posts 405.
FIG. 6 shows an estimated thermal map in a computer assembly corresponding to FIG. 4 ten seconds after maximum power was released. When power is applied to the microprocessor and memory, the temperature rises non-uniformly across various components as illustrated by FIG. 6.
FIG. 7 shows an estimated thermal map for the corresponding deformed state of the system due to thermo-mechanical interaction between the heat sink and the adjoining structure. The deformation creates distortion of TIM gap which can lead to TIM failure. Similarly, the solder joints (not illustrated) are also subjected to cyclic strain.
Heat sinks are widely used by the industry and various methods of supporting them have been disclosed in the prior art. One conventional device suggests plastically formed supports with locking features so that heat sink could be easily removed to rework a computer in the field. Another conventional device envisages the use of flexible interposing elements between the solder joints and solder pads that would reduce the strain in the solder due to thermally induced deformation. Another conventional device describes a method to reduce stress between the die surface and heat sink surface by means of cantilever means embedded within a TIM material. The cantilever beams do not support the weight of a heat sink. Another conventional device introduces compliant support of a heat sink in order to manage the preload applied to the TIM material. However, in the prior art, no consideration is given to the thermomechanical coupling effects or methods to mitigate it. The majority of the prior art covering heat sink mounting focuses on preload control and ease of removability.